Repetitive strain injuries (RSI) affect thousands of people and cost the US economy more than $14 billion each year. It has long been believed that improper postures or movements made during repetitive tasks increase the risks of developing RSI. Muscle fatigue may be an important intermediary factor in this process, since muscle fatigue can induce changes in coordination that generate improper movements, which may in turn increase the risk of RSI over time. The purpose of this R21 application is to develop and test the ability of new methods to track the changes that occur in both muscle function and coordination during fatiguing repetitive movements. A device will be constructed to simulate an upper extremity repetitive task known to induce changes in coordination after fatigue. Appropriate analytical tools for tracking fatigue from observed changes in coordination will also be developed by extending existing nonlinear dynamics algorithms developed for tracking damage accumulation in mechanical systems, Because this approach tracks distortions in appropriately reconstructed state spaces, it can provide valid measures of the underlying (hidden) damage dynamics without the need for detailed physics-based mathematical models of either the system or damage dynamics. Currently available algorithms will be modified to account for the most prominent differences between mechanical and biological systems: noise, multiple time scale dynamics, and non-monotonic damage dynamics (i.e. biological adaptability). Finally, these and more traditional measures will be applied to explore the time courses of changes in muscle function and motor coordination that occur during the repetitive work task. 30 healthy subjects will perform the task until voluntary exhaustion under three conditions: more restricted, less restricted, and less restricted at elevated work height. It is hypothesized that (1) changes in local muscle fatigue will precede changes in muscle coordination, which will in turn precede overt changes in kinematics, (2) this sequence of events will be delayed in the less restricted condition, (3) these changes will occur more rapidly in the elevated work height condition, and (4) the nonlinear tracking approaches will reveal subtle changes in coordination that reflect underlying (hidden) changes in muscle fatigue state. This project will generate new insights into the nature and time course of the biomechanical and neural adaptations that occur during repetitive tasks and will provide the necessary foundation for developing improved diagnostic techniques to identify early-onset (pre-clinical) RSI in future work. It is hoped that these efforts will one day help reduce the tremendous personal and monetary costs associated with these injuries.